1. Field of the Invention
The invention relates generally to the field of downhole oil/water separation systems. More specifically, the invention relates to automatic operation of a downhole oil/water separation system to maintain preferred system operating parameters.
2. Background Art
Hydrocarbon production systems known in the art include combinations of electric submersible pump (“ESP”) and downhole oil water separator (“DOWS”). In an ESP/DOWS production system, the ESP and DOWS are disposed in a wellbore drilled through subsurface formations. The wellbore typically has a steel pipe or casing disposed therein extending from the Earth's surface to a depth below the deepest subsurface formation from which fluid is to be withdrawn or injected.
The ESP is typically a centrifugal pump rotated by an electric motor. The intake of the ESP is in hydraulic communication with one or more of the subsurface formations from which fluid is withdrawn (the “producing formation” or “producing zone”). The ESP outlet or discharge is in hydraulic communication with the inlet of the DOWS. The DOWS has two outlets, one for water separated from the fluid withdrawn from the producing formation and the other outlet for the fluid remaining after water separation. Typically, the separated water outlet is in hydraulic communication with one or more of the subsurface formations that are used to disposed of the separated water (the “injection formation” or “injection zone”).
The DOWS is typically a hydrocyclone separator or a centrifuge-type separator. A hydrocyclone separator includes devices that cause the fluid flowing therein to move in rotational path at high speed, so as to cause the more dense water to move toward the radially outermost portion of the separator. The less dense fluid, consisting primarily of oil, is constrained to move generally along the radial center of the separator. A centrifuge separator is typically operated by a motor, which may be the same or different motor than the one that drives the ESP. Devices in the centrifuge use the rotational energy of the motor to cause the fluids entering the centrifuge to rotate at high speed, whereupon the water and oil are constrained in a manner similar to that of a hydrocyclone separator.
In order to obtain the most benefit from an ESP/DOWS production system, it is desirable to operate the ESP so that the amount of fluid moving through the ESP/DOWS system is equal to the rate at which the producing formation can produce the fluid. It is also desirable to control operation of the DOWS such that the amount of fluid injected into the injection formation is not more than the injection formation can accept, or, alternatively, that the fluid flow rate through the DOWS does not exceed the separation capacity of thereof. In the latter case oil may be discharged through the water outlet and disposed of in the injection formation.
It is known in the art to automatically control the operating rate of the ESP to cause the ESP to move a suitable amount of fluid. See, for example, U.S. Pat. No. 5,996,690 issued to Shaw et al. The system disclosed in the Shaw et al. '690 patent does not provide for any control over the fluid output from the DOWS or any separate control over the rate of fluid discharged from the water outlet of the DOWS.